The present invention relates to a process and an apparatus for separating solids from a liquid phase.
The separation of solids from a liquid phase is a task which frequently has to be carried out in industry. An example which may be mentioned is the separation of aluminum hydroxide crystals from the basic mother liquor. In the separation, it is important to free the solid as completely as possible of the liquid phase without large amounts of dilute washing liquor being obtained.
A particular separation problem occurs when the liquid phase has to be separated virtually completely ( greater than 99%) from the solid. The liquid phase frequently contains nonvolatile components which have to be separated off. In this case, drying of the solid does not solve the separation problem. Rather, the solid needs to be washed without dilute washing liquors being obtained. This is the case in the preparation of some high-performance polymers in high-boiling solvents. The process of the invention is of particular relevance to polymers which are prepared in a high-boiling solvent with formation of salts or acids or bases which have to be neutralized subsequently. Examples of such processes are the preparation of polyaramides from diamines and diacid chlorides, of polycarbonates from diphenols or diphenoxides and phosgene, of polysulfones, polyether sulfones or polyether ketones from diphenoxides and dihalogenated aromatic hydrocarbons or the preparation of polyarylenesiloxanes from diaryldichlorosilanes and diphenoxides. In particular, the term high-performance polymers encompasses polyarylene sulfides.
In the preparation of poly(p-phenylene) sulfide (PPS), the polymer has to be freed of high-boiling N-methylpyrrolidone (NMP) and undissolved and dissolved by-products. The proportion of residual solvent (NMP) in the polymer should be less than 100 ppm. The generation of dilute NMP-containing washing liquors would require a high outlay for distillation and therefore has to be avoided.
According to the prior art, separation can be carried out by means of filtration and subsequent washing of the filter cake. However, substantial volumes of washing liquid are necessary to achieve virtually complete removal of the liquid phase from the solid and have to be worked up at considerable cost.
Vaporization of the solvent from the solid/liquid mixture, if appropriate at elevated temperature, is likewise known. In the case of high-boiling solvents such as NMP, this is time-consuming and costly. In addition, nonvolatile impurities remain in the PPS solid and have to be removed in a further process step.
From an economic point of view, carrying out the abovementioned separation task in a continuously operating separation apparatus continues to be of interest.
It is an object of the present invention to provide a process and an apparatus for separating solids from solid/liquid mixtures, with the liquid phase being separated off from the solid virtually completely without a large amount of dilute washing liquor being obtained.
The object of the present invention is achieved by the solid in a first liquid phase being brought into contact with a second liquid phase in countercurrent and the solid being transferred into the second liquid phase.
The process of the invention is carried out by bringing the solid in the first liquid phase into contact with a second liquid in countercurrent in an apparatus which has at least one solid/liquid inlet (1), at least one solid/liquid outlet (4), at least one liquid inlet (3) and at least one liquid outlet (2) (cf. FIG. 1 and FIG. 3).
The solid in the first liquid phase is fed into the solid/liquid inlet (1). The second liquid phase is fed into the liquid inlet (3). The solid is almost completely discharged in the second liquid phase through the solid/liquid outlet (4). The first liquid phase is virtually completely discharged through the liquid outlet (2).
In the process of the invention, only small amounts of the first liquid phase are discharged together with the solid in the second liquid phase through the solid/liquid outlet (4). The proportion of first liquid phase, based on the second liquid phase, in the solid/liquid mixture discharged through the outlet (4) is less than 50% by weight, preferably less than 10% by weight and particularly preferably less than 1% by weight.
In the process of the invention, the first liquid phase is not greatly diluted with the second liquid phase. The proportion of second liquid phase, based on the first liquid phase, in the liquid discharged through the outlet (2) is less than 80% by weight, preferably less than 50% by weight, particularly preferably less than 30% by weight.
In general, only small amounts of solid are carried out in the liquid discharged through the outlet (2) in the process of the invention. This proportion of solid is, based on the solid introduced through the solid/liquid inlet (1), preferably less than 30% by weight, particularly preferably less than 10% by weight.
In the process of the invention, the solid travels through the apparatus by sedimentation under gravity. In a preferred embodiment of the invention, the solid of the solid/liquid mixture is sedimented at least once in an apparatus and the sedimented solid is washed in countercurrent with a second liquid phase.
In the process of the invention, the first liquid phase is preferably an organic solvent and the second liquid phase is preferably water. Organic solvents used are, for example, dipolar aprotic solvents, preferably NMP.
In the process of the invention, the first and second liquid phases are preferably miscible.
The solid can consist of one or more components. The main constituent of the solid is preferably a polymer such as a polyarylene sulfide, very particularly preferably PPS.
The process of the invention can also separate mixtures of solids it the constituents have different solubilities in the first and second liquid phases.
Preference is given to the main constituent of the solid dissolving neither in the first liquid phase nor in the second liquid phase. Secondary constituents of the solid or of the first liquid phase which are to be separated off preferably have a good solubility in the second liquid phase.
In a particularly preferred embodiment, a polymer which is to be separated off and to be purified, for example PPS, is insoluble in the first and second liquid phases under the conditions of the process of the invention, while the impurities to be removed, for example salt and high-boiling solvent, dissolve in the second liquid phase.
A particular advantage of the process of the invention is that it offers the opportunity of the separating off the solid continuously even at elevated temperature and under superatmospheric pressure.
In the process of the invention, the density of the solid is different from the density of the first liquid phase and of the second liquid phase.
The object is achieved by a process in which the apparatus of the process of the invention comprises at least one top section, at least one bottom section and at least one middle section having n sedimentation trays.
A first preferred embodiment of the invention is a process using an apparatus which has at least one sedimentation tray (ST) which comprises at least one sedimentation funnel having a sediment outlet, at least one flow zone and at least one supernatant liquid outlet.
The solid is sedimented on each sedimentation tray in the sedimentation funnel and washed in the flow zone. This is repeated sequentially in n stages. The sedimentation tray has a funnel-like construction and ends in a bottom outlet which represents the solid/liquid outlet. The top section has an inlet for the solid/liquid mixture which leads to the uppermost sedimentation tray. The top section also has an outlet for discharge of the supernatant liquid from the uppermost sedimentation tray.
The apparatus of the process of the invention makes it possible to separate the solid from the mixture in such a way that the proportion of the first liquid phase which is carried out with the solid can be reduced by increasing the number of sedimentation trays.
The sedimentation trays are arranged above one another. The supernatant liquid outlet of one sedimentation tray is connected to the flow zone of the next sedimentation tray above it. The supernatant liquid of a sedimentation tray comprises a mixture of the first and second liquid phases. In the flow zone, the supernatant liquid passes through the sedimenting solid in a precisely defined geometric arrangement and effects at least partial displacement washing of the solid. In this way, the supernatant liquid of one sedimentation tray is used as washing liquid for the sedimentation tray above it. Fresh washing liquid is fed into the flow zone of the bottommost sedimentation tray through the liquid inlet (3) in the form of pure second liquid phase. While the solid passes through the apparatus of the process of the invention from the top downward, the washing liquid flows from the bottom upward. This countercurrent principle enables extraordinarily effective separation of the first liquid phase from the solid to be achieved.
The geometric arrangement of the flow zone is selected so that the solid does not sediment in a direction directly opposite to the flow of the ascending washing liquid in the individual sedimentation trays. The passage of the solid into the next sedimentation tray is not hindered by the washing liquid which passes through the apparatus in countercurrent, since the washing liquid is conveyed through separate supernatant liquid outlets to the flow zone of the next higher sedimentation tray. In this way, largely unimpeded sedimentation is achieved, as a result of which solids having a low sedimentation velocity can also be separated off.
The separation efficiency of the apparatus of the process of the invention depends essentially on the number of sedimentation trays. The separation efficiency can be matched to the respective separation task via the number of sedimentation trays. The separation efficiency increases with increasing number of sedimentation trays.
A preferred embodiment of the invention is an apparatus having n sedimentation trays, where n is an integer from 2 to 1000.
A particularly preferred embodiment of the invention is an apparatus in which n is an integer from 3 to 200.
A very particularly preferred embodiment of the invention is an apparatus in which n is an integer from 4 to 100.
A second preferred embodiment of the invention is a process using an apparatus which comprises a straight tube having a centered stirrer shaft. The stirrer shaft bears a cross-arm agitator having at least one level. A plurality of stirrer blades or stirrer arms are arranged in one horizontal plane of the cross-arm agitator.
While the solid entering through the solid/liquid inlet travels through the apparatus of the process of the invention from the top downward, the washing liquid flows from the bottom upward. By means of displacement of the first liquid phase by the second liquid phase, which increases continuously from the top downward, it is possible to separate the solid from the mixture in such a way that the proportion of the first liquid phase which is carried out with the solid can be reduced. Fresh washing liquid is fed to the apparatus at the lower end of the middle section through the liquid inlet (3) in the form of pure second liquid phase. As a result of this countercurrent principle, extraordinarily effective separation of the first liquid phase from the solid is achieved.
The geometric arrangement of the cross-arm agitator is chosen so that it largely suppresses stream formation in the ascending liquid phase 2. In particular, the stirrer blades reach almost to the inside wall of the straight tube. Stream formation would impair the washing efficiency of the process of the invention. At least two stirrer blades or stirrer arms per level of the cross-arm agitator have been found to be useful. In a preferred embodiment, the cross-arm agitator bears from three to eight, particularly preferably four, stirrer blades or stirrer arms per level.
The separation efficiency of the apparatus of the process of the invention depends essentially on the number and spacing of the levels of the cross-arm agitator. The separation efficiency can be matched to the respective separation task via the number of levels. The separation efficiency increases with increasing number of levels.
During operation, the cross-arm agitator turns at a moderate speed. The speed of rotation is typically from 0.1 to 1000 rpm, preferably from 1 to 200 rpm and particularly preferably from 5 to 50 rpm.
It is frequently advantageous for the uppermost sedimentation tray or the top section of the apparatus to have a greater diameter than the sedimentation trays lower down. This measure reduces the proportion of solid in the liquid discharged through the outlet (2) which forms the supernatant liquid outlet of the uppermost sedimentation tray.
Suitable solid/liquid mixtures are mixtures in which the solid sediments under the force of gravity. The upper limit to the solids content is determined by the flowability of the mixture. The apparatus can separate off very small proportions of solid too.
The density of the solid differs from the density of the liquid phase. The sedimentation velocity of the solid in the liquid phase should be greater than 1 mm/min. The velocity at which a solid sinks is dependent on the density different between it and the liquid phase, the mean particle size, the particle size distribution and the viscosity of the liquid phase. The apparatus of the process of the invention allows solids having particle sizes of  greater than 1 xcexcm to be separated off. The separation is improved by increasing particle size and an increasing density difference.
The separation efficiency of the apparatus of the process of the invention can be matched to the respective separation task within wide limits by changing the construction of the apparatus and adjusting the operating conditions. In a first preferred embodiment, the construction of the apparatus can be altered by adjustment of the number of sedimentation trays n, the inclination of the sedimentation funnels, the geometry of the sediment outlet and of the supernatant liquid outlet and also the geometric configuration of the flow zone. In a second preferred embodiment, the construction of the apparatus can be altered by adjustment of the number n of the levels of the cross-arm agitator, the vertical spacing of the levels and the shape of the stirrer blades or stirrer arms.
Advantageous operating conditions are achieved by adjusting the inflow and outflow rates of solid/liquid mixtures, the inflow rate of the second liquid phase or the rotational speed of the cross-arm agitator.
It is also possible to use a plurality of apparatuses connected in series. This arrangement improves the separation efficiency for a given throughput.
The invention will now be illustrated with the aid of a drawing and an example.